gianfar: Fix warnings when built on 64-bit

As part of defconfig consolidation using fragments, we'd like to be
able to have the same drivers enabled on 32-bit and 64-bit.  Gianfar
happens to only exist on 32-bit systems, and when building the
resulting 64-bit kernel warnings were produced.

A couple of the warnings are trivial, but the rfbptr code has deeper
issues.  It uses the virtual address as the DMA address, which again,
happens to work in the environments where this driver is currently
used, but is not the right thing to do.

Fixes: 45b679c9 ("gianfar: Implement PAUSE frame generation
support")
Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>